Method of controlling the hydrogenation of acetylene

ABSTRACT

1. IN A CONTINUOUS PROCESS FOR THE HYDROGENATION OF ACETYLENE TO ETHYLENE IN A CONVERSION ZONE; A METHOD OF CONTROL WHICH COMPRISES DETERMING THE CONCENTRATION OF HYDROGEN IN THE VAPOROUS EFFLUENT WITHDRAWN FROM SAID CONVERSION ZONE, ADJUSTING THE TEMPERATURE OF THE FEED MOIXTURE CONTAINING ACETYLENE, ETHYLENE AND HYDROGEN PASSED TO SAID CONVERSION ZONE RESPONSIVE TO SAID HYDROGEN CONCENTRATION, DETERMINING THE CONCENTRATION OF ACETYLENE IN SAID VAPOROUS EFFLUENT, AND ADJUSTING THE CONCENTRATION OF HYROGEN IN THE VAPOROUS FEED TO SAID CONVERSION ZONE RESPONSIVE TO SAID ACETYLENE CONCENRATION AND   THE TRETEMPERATURE TO WHICH SAID VAPOROUS FEED IS TO BE HEATED RESPONSIVE TO SAID HYDROGEN CONCENTRATION.

United States Patent O 3,839,483 METHOD OF CONTROLLING THE HYDROGENA- TION OF ACETYLENE Norman L. Carr, The Hague, Netherlands, and Malcolm D. Fraser, Pittsburgh, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa.

Filed Jan. 29, 1973, Ser. No. 327,591 Int. Cl. C07c 11/00 US. Cl. 260-677 A 5 Claims ABSTRACT OF THE DISCLOSURE A method of controlling the hydrogenation of acetylene to ethylene which comprises l) adjusting the ratio of hydrogen to acetylene in a feed to a hydrogenation zone responsive to the concentrations of hydrogen and acetylene in the eflluent, and (2) adjusting the temperature of the acetylene feed to the hydrogenation zone responsive tothe hydrogen concentration in the eflluent.

BACKGROUND OF THE INVENTION The conversion of minor concentrations of acetylene to ethylene in, for example, ethylene product streams is conventionally eifected by hydrogenating the acetylene in the presence of a suitable hydrogenation catalyst. It is desirable to completely hydrogenate the acetylene under conditions that will not encourage the hydrogenation of ethylene and will minimize the consumption of hydrogen. The difficulty of converting acetylene to ethylene under such conditions is intensified when the concentration of acetylene is below 1.0 weight percent.

Accordingly, an object of the invention is to provide an improved process for hydrogenating acetylene to ethyl ene.

Another object of the invention is to provide an improved process for hydrogenating acetylene to ethylene in an ethylene-acetylene feed mixture.

Yet another object of the invention is to provide an improved method for controlling the hydrogenation of acetylene to ethylene while minimizing the hydrogenation of ethylene.

Other objects, advantages and features of this invention will be readily apparent to those skilled in the art from the following description, drawing and appended claims.

SUMMARY OF THE INVENTION In the hydrogenation of acetylene, an improved control system is provided which comprises adjusting the ratio of hydrogen to acetylene in the feed to the hydrogenation zone responsive to the concentration of acetylene and hydrogen in the hydrogenation zone effiuent, and adjusting the temperature of the feed to the hydrogenation zone responsive to the hydrogen concentration in the hydrogenation zone efiluent so as to completely hydrogenate the acetylene while minimizing the consumption of hydrogen and the hydrogenation of ethylene.

DESCRIPTION OF THE DRAWING The drawing is a schematic representation of the inventive acetylene hydrogenation control system.

DESCRIPTION OF THE INVENTION The inventive control system is applicable generally to acetylene hydrogenation processes but finds particular application in those hydrogenation processes where the con- 7 centration of acetylene in the hydrocarbon feed is low and it is desirable to minimize the hydrogen feed to the hydrogenation zone. Although not to be limited thereto, the invention will hereafter be described with reference to the hydrogenation of acetylene in an ethylene feed where the concentration of acetylene is less than 1.0 mole percent.

3,839,483 Patented Oct. 1, 1974 In the description of a specific embodiment of the invention, reference is made to the drawing where a vaporous feed mixture comprising ethylene, ethane, and acetylene is passed continuously via conduit 10 to a heat exchange means 11 such as a steam heater. Hydrogen is passed via conduit 12 to conduit 10 wherein the hydrogem is mixed with the vaporous hydrocarbon feed.

The heated vaporous mixture is passed from heat exchange means 11 via conduit 15 to a converter 13 wherein the vaporous mixture is contacted with a conventional hydrogenation catalyst such as palladium-on-alumina. It will be understood by those skilled in the art that the temperature to which the vaporous mixture is preheated will depend upon the particular hydrogenation catalyst employed and the space velocity utilized in the hydrogenation process.

An efiluent mixture comprising ethylene, hydrogen and possibly unconverted acetylene is withdrawn from converter 13 via conduit 14. It is desired to hydrogenate all of the acetylene in the feed to ethylene in converter 13 under conditions such that the hydrogenation of ethylene to ethane is eliminated and the concentration of hydrogen required in the feed to converter 13 is minimized.

In order to maintain these desired objectives, the novel control system comprises determining the hydrogen concentration in the efiluent mixture employing a conventional H analyzer 16 and transmitting a signal representative of the ditlerence between the measured and required hydrogen concentration as a set-point adjustment to a conventional flow controller 17. If the hydrogen concentration in the efiluent is below that established as required to hydrogenate all of the acetylene in the feed to ethylene, a set-point signal is transmitted to a conventional flow controller 17 for the purpose of reducing the temperature to which the vaporous feed to converter 13 is heated. This reduction in heating is accomplished by adjusting the flow of the heat exchange medium passing through valve 18 responsive to the lowered set-point signal and the temperature of the vaporous feed flowing through conduit 15 as measured by a conventional temperature indicator 19. If the hydrogen concentration is above that required for hydrogenation of the acetylene, a set-point signal is transmitted to flow controller 17 resulting in an increase in the flow of heat to heat exchange means 11 and an increase in temperature of the vaporous feed in conduit 15.

The signal responsive to the hydrogen analysis in the eflluent mixture is also passed to a summing device 20 wherein it, is combined with a hereafter described signal received from an acetylene analyzer 21.

The concentration of acetylene in the eflluent mixture passing through conduit 14 is determined by a conventional acetylene analyzer 21. A signal responsive to the difference between the measured and desired acetylene concentration is passed to a summing means such as a con-- ventional adding relay 20, wherein it is added to the signal transmitted to ratio flow controller 22. A signal is transmitted by adding relay 20 to ratio flow controller 22 as a set-point adjustment responsive to the combination. In so combining the signals, the set-point signal transmitted by adding relay 20 is responsive to the acetylene concentration in the efliuent and, via the hydrogen concentration in the effluent, the temperature to which the vaporous feed to converter 13 is to be heated. Combining the signals and transmitting a set-point signal to ratio flow controller 22 responsive thereto comprises a combined feedforward and a feedback control step. Ratio flow controller 22 opens and closes valve 23 responsive to the set-point signal, rate of flow measurement in conduit 10 as seen by flow recorder 24, and rate of flow measurement in conduit 18. It will be appreciated by those skilled in the art that the effect of a change in the hydrogen or acetylene concentration in the efiluent as seen by adding relay 20 may be time-delayed to compensate for process deadtime as measured, for example, by the time required before a change in temperature adjustment through opening or closing valve 18 can become elfective.

Assuming that the acetylene analysis of the efiluent mixture indicates that additional hydrogen is required in converter 13 to hydrogenate the acetylene to ethylene, the signal transmitted by acetylene analyzer 21 to adding relay 20 Would result in a set-point signal being transmitted to ratio flow controller 22 so as to increase the concentration of hydrogen in the vaporous mixture feed to converter 13. Hydrogen analyzer 16 would pass a signal representative of the hydrogen concentration to adding relay 20 wherein the signal would be combined with the signal received from acetylene analyzer 21. The effect of the signal received from hydrogen analyzer 16 under the circumstances presented would be to increase or decrease the rate of flow of hydrogen to converter 13 in a feedforward manner in addition to that dictated by the concentration of acetylene should the hydrogen concentration be higher or lower, respectively, than desired in the eflluent mixture.

The method of control outlined above assumes that the selectivity of the catalyst for the conversion of acetylene to ethylene decreases with an increase in the temperature of the vaporous feed to converter 13. Under such conditions, if the inlet temperature of the vaporous feed is to be increased to control the hydrogen in the effluent, the hydrogen ratio in the feed must be increased to counteract the effect of the inlet temperature on the acetylene conversion. Conversely, if the vaporous inlet temperature is to be decreased to control hydrogen in the efiluent the hydrogen ratio in the feed must be decreased to counteract the elfect of the inlet temperature on the acetylene conversion.

The described method of control is particularly effective in acetylene conversion processes wherein it is desired to reduce the concentration of acetylene, Without hydrogenation of ethylene to ethane, in the effluent mixture to less than parts per million (p.p.m.). It has been observed that these objectives can be obtained while maintaining concentrations of hydrogen in the eflluent mixture substantially less than 100 p.p.m., normally less than 5 p.p.m.

The following example is presented to illustrate the objects and advantages of the invention. It is not intended, however, that the invention be limited to the specific embodiments described therein.

EXAMPLE A vaporous feed mixture comprising 65.0 mole percent ethylene, 34.5 mole percent ethane, and 0.5 mole percent acetylene is continuously passed at a temperature of 100 F. over a palladium-on-alumina-catalyst at a rate of .26 hour/lb./lb. Hydrogen is passed to the acetylene conversion zone at a rate such that the mole ratio of hydrogen to the total flow of feed is 0.007. The analysis of the efiluent withdrawn from the acetylene conversion zone indicates a concentration of acetylene of 3.5 p.p.m. and a concentration of hydrogen of 20 p.p.m.

To demonstrate the elfectiveness of the control system as described in the drawing, the concentration of acetylene in the vaporous feed is increased by percent or to a level of 0.55 mole percent of the vaporous feed. Within 5 minutes after the increase of acetylene in the efiluent mixture to 7.5 p.p.m. is noted, the rate of flow of hydrogen through conduit 12 is increased so as to increase the hydrogen to feed ratio to 0.085. Analysis of the efiluent mixture indicated that the concentration of hydrogen has decreased to 17 p.p.m.

Within 7 minutes the inventive control system operated to reduce the concentration of acetylene in the efiluent mixture to 3.2 p.p.m. and the hydrogen concentration in the eflluent mixture had been increased to 20 p.p.m.

Although the invention has been described with reference to specific embodiments, references, and details, various modifications and changes will be apparent to one skilled in the art and are contemplated to be embraced in the invention.

We claim:

1. In a continuous process for the hydrogenation of acetylene to ethylene in a conversion zone; a method of control which comprises determining the concentration of hydrogen in the vaporous efiluent withdrawn from said conversion zone, adjusting the temperature of the feed mixture containing acetylene, ethylene and hydrogen passed to said conversion zone responsive to said hydrogen concentration, determining the concentration of acetylene in said vaporous efiluent, and adjusting the concentration of hydrogen in the vaporous feed to said conversion zone responsive to said acetylene concentration and the temperature to which said vaporous feed is to be heated responsive to said hydrogen concentration.

2. The method of control of Claim 1 wherein the temperature of the feed mixture passed to said conversion zone is lowered when the concentration of the hydrogen in said eflluent is less than desired.

3. The process of Claim 1 wherein the temperature of the feed mixture passed to said conversion zone is elevated when the concentration of hydrogen in said effluent is above the desired concentration.

4. The process of Claim 1 wherein the concentration of acetylene in said vaporous feed is less than 1 mole percent.

5. The process of Claim 4 wherein the hydrogenation process is conducted in the presence of a palladium-onalumina-catalyst and the concentration of acetylene in said effiuent is less than 5 p.p.m.

References Cited UNITED STATES PATENTS 3,116,342 12/1963 Robinson et al. 260677 3,471,582 10/1969 Lupfer 260677 H 3,549,720 12/1970 Wright et a1 260677 H FOREIGN PATENTS 655,603 6/ 1960 Canada 260677 A DELBERT E. GANTZ, Primary Examiner J. M. NELSON, Assistant Examiner US. Cl. X.R.

208DIG l, 143; 260677 H 

1. IN A CONTINUOUS PROCESS FOR THE HYDROGENATION OF ACETYLENE TO ETHYLENE IN A CONVERSION ZONE; A METHOD OF CONTROL WHICH COMPRISES DETERMING THE CONCENTRATION OF HYDROGEN IN THE VAPOROUS EFFLUENT WITHDRAWN FROM SAID CONVERSION ZONE, ADJUSTING THE TEMPERATURE OF THE FEED MOIXTURE CONTAINING ACETYLENE, ETHYLENE AND HYDROGEN PASSED TO SAID CONVERSION ZONE RESPONSIVE TO SAID HYDROGEN CONCENTRATION, DETERMINING THE CONCENTRATION OF ACETYLENE IN SAID VAPOROUS EFFLUENT, AND ADJUSTING THE CONCENTRATION OF HYROGEN IN THE VAPOROUS FEED TO SAID CONVERSION ZONE RESPONSIVE TO SAID ACETYLENE CONCENRATION AND 